Process for producing silicon steel with preferred orientation



Oct. 13, 1964 G. w. WlENER ETAL 3,152,929

PROCESS FOR PRODUCING SILICON STEEL WITH PREFERRED ORIENTATION Filed Aug 17, 1959 a 4 6 Chlorine Pressuro- MM of Hg (IBOOC) M. M R m Y 08C N T N60 R E c 0 w w T Wt A r 86 W mY 0 2 G j 3 w .W V F O m 3 I M I Q n 4 m I r c M m a M h 0 0 C u U u [m m H P .0 .M M m 5U 40 e r P o o o o w w 3 2 United States Patent 3,152,929 PROCESS FGR PRGDUCING SILECGN STEEL WITH PREFERRED GRIENTATION George W. Wiener, (Zhurchiil Born, and Robert W. Corcoran, North Versailles Township, Allegheny County, la assignors to Westinghouse Electric Corporation, East Pittsburgh, Pa, a corporation of Pennsylvania Filed Aug. 17, 1959, Ser. No. 834,042 Claims. (Cl. 148-111) This invention relates to a process for producing silicon steel sheets with a relatively high proportion of cube-onface grains.

It has been proposed heretofore to produce silicon steel sheets having a double orientation resulting from a substantial proportion of cube-on-face grains. The cube-onface or double orientation grain orcrystal structure in magnetic sheets comprises grains in which the cube faces are substantia lly parallel to the surface of the sheet and the cube edges are substantially parallel to the direction of rolling and also crosswise to the direction of rolling or sheet edge. Such grains are identified in Miller Indices as being of the (100) .[001] type.

As is well known to those skilled in the art, the permeability and other magnetic properties are outstanding in the rolling direction or the [100] direction of the grains since this is the direction of easiest magnetization thereof. The direction of easiest magnetization of a cube grain is along the cube edges, magnetization is more diflicult along any cube face diagonal and the most difficult along the long cube diagonals. Therefore, in any other direction other than along the rolling direction, as, for example, the transverse direction of the sheet, the magnetic properties of the singly oriented sheets are greatly inferior because the magnetization is not parallel to an edge of the cube.

' If sheets of -a cube-on-face or double oriented grain texture were available so that a high proportion of the grains had two of their cube faces parallel to the planes of the. sheet with their. cube edges closely parallel both to the rolling direction and to the crosswise direction of the sheet, the magnetic properties of such sheets would be outstanding both in the rolling direction of .the sheet and in the transverse direction of the sheet. It'would be highly desirable to have available sheets in which over 70%, and preferably 90% and higher, of the grains had cube faces within 5 to of the plane of the sheet, and it would, be further desirable if the texture of the grains were sharp, that is, the cube edges were closely parallelto one another, and over 75%, for instance, were within to of the direction of rolling. 'One of the difliculties that has been encountered is the lack of consistency in producing sheets of silicon steel having a high proportion of cube-on-face grains.

Not all of the crystal structure of the sheets of silicon steel that are subjected to a given -treatment converts to a high and-uniform proportion of cube-on-face grains, often only a small portion of the grains resulting from a final anneal havea (100) [001] orientationfor example, only 20% to 50% of the grains may be so oriented.

In other .casesthe grains in a given sheet may have. a

texture is a problem also. As pointed out in US. application, Serial No; 788,596, of George W. Wiener, one

of the co-inventors of the present application, filed Ianuary 23, 1959, and assigned to the. assignee of the present invention, there has been acon siderable' variation in the amount of cube-on-face grains produced during the final anneal in any particular silicon steel sheet. It would be desirable to have available a process that would assure the steel manufacturer of producing silicon steel sheets having a very high proportion of cube-on-face grains, and, of course, with a sharp texture so that the cube face is parallel to the sheet surface and the cube edges of most of these grains closely parallel to the direction of rolling.

The object of the present invention is to provide a process in which cold rolled sheets of silicon steel are annealed at an elevated temperature to provide grain growth by secondary crystallization under conditions in which the annealing atmosphere will produce a bright metallic surface and said atmosphere contains a vapor of a material which will accelerate cube-on-face grain growth, so that there will be consistenly produced a high proportion of grains having cube-on-face orientation.

A further object of the invention is to provide a final annealing process for treating cold rolled silicon iron sheets at a temperature .of from 1100 C. to 1425 C. in an atmosphere which contains vapors at a low vapor pressure of an accelerator for cube-on-face grain growth, which accelerator reacts with the silicon iron surfaces to form transient but not stable reaction products, or interacts with the metal surface in such a way as to change the rates of reactions normally occurring during the heat treatment.

' Other objects of the invention will, in part, be obvious and will, in part, appear hereinafter.

For a better understanding of the nature and objects of the invention, reference should be had to the following detailed description and drawing, in which:

FIGURE 1. is a graph plotting the partial pressure of chlorine gas in an annealing atmosphere against the proportion of cube-on-face grains;

FIG. 2 is a graph plotting the partial pressure of argon and bromine vapors against proportion of cube-on-face grains;

FIG. 3 is a graph plotting the partial pressure of chlorine and bromine against proportion of cube-on-face grains.

In accordance with the present invention, it has been discovered that sheets of cold rolled silicon iron alloys having from 1% to 10% silicon may be subjected to a final anneal in the presence of a vapor of a compound or element which will accelerate the growth of cube-onface grains, The vapors may have a partial pressure as low as 0.1 millimeter of mercury to as much as millimeters of mercury. The optimum partial pressure of the vapor will depend in part on the material comprising the vapor, and on the annealing temperature, the size of the charge being annealed, and whether the system is dynamic or, static. These vapors preferably will comprise at least one element from the group consisting of halogens, namely, chlorine, bromine and iodine, and the hydrogen chloride, hydrogen bromide and hydrogen iodide compounds thereof which Will dissociate at the annealing temperatures to produce vapors of the element itself.

More specifically, the present invention relates to the annealing of cold rolled silicon iron sheets having the following composition. The silicon may comprise from l%'to 10%, and preferably will be from 2 to 6%. The carbon in general will be less than 0.01%. The alloys will usually contain-manganese in the proportions of from 0.01% to 0.5 Other additives such as scavengers and impurities, such as sulfur, phosphorus and oxygen, in an amount not exceeding 0.5%, may be present. It will be understood that the alloy when in the form of cast ingots may have present therein carbon in an amount substantially greater than the limit of 0.01%. During the hot rolling and annealing, the carbon content will be reduced considerably, particularly if wet hydrogen annealing is 3 employed during the process. of course, is iron.

The invention may be carried out by hot rolling the iron-silicon alloy ingots to plates of a thickness of from about 0.10 to 0.50 inch in one or more stages, then cold rolling the plates to desired gauge in one or more stages with intermediate anneals at from 750 to 1000 C. in wet or dry hydrogen. The final rolling also can be carried out on single oriented cold rolledsheets that may be prepared in any suitable manner. It is only necessary to employ a sheet of silicon steel which is of such a thickness that when rolled to the desired gauge it will have been cold reduced from about 50% to 95% of its thickness, and preferably 70% to 85%, for the final coldrolling step. The cold reduction, which may be elfectcd at room temperature, in practice may cause the sheets to heat up to temperatures of as high as 200 C. to 400C. It is necessary that the cold reduced sheets be substantially free from any adherent surface films or coatings. However, small amounts of oxides may be present as discontinuous inclusions or particles.

The process of the present invention may be employed to produce double oriented silicon iron magnetic sheets of a thickness from 0.1 to 30 mils. Outstanding results are obtained when'applied to sheets of a thickness of from 5 to 25 mils. f

The cold reduced sheets may be annealed either as a single continuous strip or sheet, though normal commercial annealing practice will dictate that an assembly be made either in coil form or as a stack comprising a number of sheets. There should be interposed between the surfaces of the sheets in such assembly, a layer of an The balance of the alloy,

inert inorganic refractory material to prevent welding of the sheets and to allow escape of gases from the metal and to allow the selected annealing atmosphere gases to penetrate to all the surfaces or to allow evacuation to degas the metal surfaces thoroughly. The inert inorganic refractory may comprise a coating of a fine ceramic powder sifted or otherwise applied to the surface of each sheet in the assembly. A finely divided powder such for example as aluminum oxide, zirconium oxide or high purity anhydrous magnesia will give good results. The refractory preferably should be pretreated, as for example, by calcining at a high temperature so that during annealing it will not evolve any moisture, oxygen or other oxidizing materials such as carbon dioxide or the like. Good results have been obtained by using as a sheet separator 100 to 350 mesh alumina that has been calcined or fired at from 1000 C. to 1400 C. and then stored in a sealed container until ready for use.

The critical features of the present invention comprise subjecting thefinally cold reduced sheets of silicon iron to a final anneal at an elevated temperature of from 1100 C. to 14-25 C. in an atmosphere which is capable of reducing silica and will give to the surface of the sheets a bright metallic appearance. Such surfaces should be free from any continuous films of oxides or other inclusions during the early stages of the annealing treatment [so that [001] grain growth will not be hindered. The

atmosphere may comprise a vacuum of an absolute pressure less than 0.1 mm. of Hg, the vacuum may be as low as mm. of Hg, or a dry hydrogen atmosphere free and the hydrogen chloride, hydrogen bromide and hydrogen iodide compounds which compounds dissociate at the annealing temperatures to produce some of the free element. The partial pressure of said vapor is in the range of from 0.1 to 100 mm. of Hg.

Unexpectedly, the higher the annealing temperature the higher the vapor pressure of the compound that may be necessary for optimum acceleration of cube-on-face grain growth. It will be understood that in the event that a vacuum is employed, the vapor of the particular element or compound will, of course, modify the absolute pressure of the vacuum to that extent. It will be understood that the elements or compounds may be employed in admixture of two or more as will be disclosed hereinafter. The annealing should be continued until secondary recrystallization takes place in the sheets and is substantially complete. The annealing should be continued until the maximum amount of cube-on-face' graingrowth has been obtained. Under the proper conditions, the

cube-on-face grain nuclei will grow and consume other crystalline configurations which have less favorable surface energy under theconditions of annealing.

The following examples are illustrative of the practice of the present invention.

Example 1 Commercial silicon iron ingots containing 3% silicon were processed by hot rolling into plates of a thickness of 0.240 inch over a temperature range of from 1300 C. to 1350 C. The resulting hot rolled plate was annealed for two hours at 900C. in hydrogen. The hydrogen had a dew point of approximately 20 C. The annealed hot rolled plate was then cold rolled to a thickness of 0.20 inch. The cold rolled plate was then annealed for three hours at 1200 C. in a vacuum of less than 0.1 mm. of Hg. The annealed plate was again cold rolled at room temperature to a thickness of 0.040 inch. Following this second cold rolling step, the sheet was annealed for two hours at 900 C. in a hydrogen atmosphere of a dew point .of 20 C. The annealed sheet was again cold rolled at room temperature to a final thickness of 0.012 inch.

, The sheet at final gauge was then placed in an iron tube furnace where it was heated in'a vacuum. of 10 mm. of Hg to a temperature of 1000 C. The furnace was then cut off from the pumping system and chlorine gas was from oxygen, oxidizing impurities and water vapor, the

dew point of the hydrogen should be at least -40 C., and

preferably '50 C. and less, at the surfaces of the sheets.

The annealing atmosphere should include a vapor of an element or compound which will accelerate cube-onface grain growth. The vapor should comprise at least one material which is reactive with the silicon iron surfaces at the annealingtemperature and at the partial pres-' introduced into the furnace to a pressure of 3 mm. of Hg. The temperature of the furnace was increased to 1300 C. and the furnace was held at this temperature for 1 hour. During this time secondary recrystallization ofthe silicon iron took place to substantial completeness. Atthe end of the hour, the furnace was allowed to cool to room temperature. Whenthe furnace temperature had reached 1000 0., the furnacewas evacuated to remove chlorine gas from the system.

The annealing procedure was repeated utilizing different' pressures of chlorine, a fresh portion of the cold rolled strip being used, for each annealing test. The resulting sheets were examined carefully for the amount of cube-on-face grains present thereon. Grains that had cube edges within 20 degrees of the rolling direction were considered to be cube-on-face grains. Nearly all of these cube-on-face grains had faces within v,5 degrees of the plane of the surface of'the sheet. 'The curve of FIG. 1 was plotted from the data obtained inthis test. It will be noted that at a partial pressure of chlorine of approximately 3 mm. of Hg there was obtained an optimum of approximately 70% of cube-on-face grains from the sheet. In the absence of chlorine, the cube-on-face grains amounted to only "some 22%. When the chlorine content exceeded 6 mm. of Hg the cube-on-face grain content was fairly constant at approximately 32% up to a partial pressure of 12 mm. of Hg.

The same results as indicated in Example I can be obtained 'by introducing dry hydrogen chloride gas at simi- Y lar vapor pressures into the furnace instead of chlorine gas. Example 11 The procedure of Example I was repeated using bromine gas in the annealing atmosphere at 1300 C. for the final anneal. The curve shown in FIG. 2 labeled bromine illustrates the eifect of bnomine at various pressures in terms of percentage of cube-on-face grains developed in the sheets. It will be noted that the optimum amount of cube-on-face grain growth is obtained at from 4% to 7 mm. of Hg pressure for bromine. The optimum content of cube-on-face grains being approximately 40 to 43% under these conditions.

Example III The process of Example I was repeated, however, the furnaceduring final anneal was maintained for one hour at a temperature of 1200 C. as the maximum temperature. Bromine and chlorine, respectively, were employed as the accelerating gases in this experiment. The results obtained by the use of chlorine and bromine are plotted as the curves in FIG. 3 of the drawing. It will be noted that at 1200 C. annealing temperatures, chlorine produced an optimum cube-on-face graingrowth at a pressure of approximately 0.2 mm. of Hg vapor pressure while the optimum cube-on-face grain growth obtained by employing bromine was at about 0.5 mm. of Hg.

' When the experiment of Example II was repeated,

however, employing a maximum annealing temperature of 1400 C. for .one hour, the optimum cube-on-face grain growth was obtained at a bromine partial pressure of approximately 10 mm. of Hg. At 1400 C. the per'-.. centage of cube-on-face grain growth was four times as great as was obtained with approximately 4 mm. of Hg of bromine. In general, when using the halogen gases or compounds containing halogen gases such 'as hydrochloric, hydrobromic and hydriodic acids, the pressure of the halogen should be less than 30 mm. of Hg at an annealing temperature of 1400 C., less than mm. of Hg, and preferably below..6 mm.,- at 1300 C. and less than 1.5 mm. of Hg at 1200 C.

, After annealing the sheets at 1300 C. for a period of 10 minutes, in a vacuum of '10- mm. of Hg, air may be introduced at a pressure of from 0.1 to 1 mm. of Hg, and improved cube grain-textureis obtained.

' Mixtures of two or more of the gases may be employed. Thus, for example, a mixture of hydrogen, bromine and chlorine may be introduced into the furnace. Similarly, chlorine at 2 mm. may be employed in the annealing atmosphere. A

It should be understood that the process of the present invention may be carried out in a hydrogen atmosphere, preferably hydrogen of a dew point of at least 40 C. The pure dry hydrogen atmosphere may be employed to initiate the reaction at the surface of the sheets to remove all silicon oxides and then the accelerator added after a suitable initial period of time. Hydrogen may be present in amounts varying from 1 mm. of Hg up to atmospheric pressure. I

'In addition, metalli vapors such as iron, nickel, chromium and alkali metals such, for example, as lithium, may be present in the vicinity of the sheets at the vapor pressure thereof at the annealing temperature in order examples above, the silicon iron sheets would have other- Wise developed less than 50% of cube on-face grains during final anneal, and the addition of the accelerator vapors to the annealing atmosphere will enable a very marked increase in the number of cube-on-face grains,

6 to 70% or even more, on complete secondary recrystallization. If the silicon-iron sheets would have developed from 50% to 70% of cube-on-face grains during the final anneal, then the addition of the accelerator vapors will enable to or even more of cube-on-face grains to be produced in the same sheets during the final annem. Furthermore, by following the procedures disclosed in copending patent application Serial No. 788,596, filed January 23, 1959, and assigned to,the assignee of the present invention, the silicon-iron sheets can be subjected to final annealing for a period of time in the presence of accelerator vapors, then removed and subjected to etching and then annealed further in the presence of accelerator vapors to secure the utmost cube-on-face grain growth. Thismaybe repeated one or more times.

'It will be understood that the above description and drawing are only exemplary.

We claim as our invention: 1. In the process of producing silicon-iron sheets having a high proportion of cube-on-face grains, the steps comprising annealing cold rolled sheets of a thickness of from 0.1 to 30 mils of silicon-iron comprising from 1% to 10% silicon, the balance being essentially iron except for small additions and incidental impurities at a temperature of from 1100" C. to l425 C..in an atmosphere which during at least the initial stages of the annealing.

will produce on the sheets a bright metallic surface, free from any continuous films, said atmosphere containing as an essential component to provide an acceleration for cube-on-face grain growth, a vapor of at least one material reactive with the silicon-iron which at the annealing temperature and at the partial pressure thereof in the annealing atmosphere is capable of forming transient but not stable reaction products on the surface of the sheets, said vapor comprising at least one halogen element from the group consisting oflchlorine, bromine and iodine, and hydrogen chloride, hydrogen bromide and hydrogen iodide which dissociate at the annealing temperatures, the partial pressure of said. vapor being in the range of from 0.1 to millimeters of mercury, and continuing the annealing until the surfaces are bright and substantially complete cube-on-face grain growth by secondary recrystallization takes place.

2. In the process of producing silicon-iron sheets having a high proportion of cube-on-face grains, the steps comprising a vacuum annealing cold rolled sheets of a thickness of from 0.1 to 30 mils of silicon-iron comprising from 1% to 10% silicon, the balance being essentially iron except for small additions and incidental impurities, at a temperature of from 1100 C.ito 1425 C. in an atmosphere which during at least the early stages of the annealing will produce on sheets a bright metallic surface free from any continuous films, said atmosphere containing as an essential component to provide an accelerator for cube-on-face grain growth a vapor of at least one material selected from the group consisting of chlorine, bromine and iodine reactivewith the silicon iron, the vapor beingpresent at a partial pressure of from 0.1 to 100 mm. .of Hg, the vapor at the annealing temperature and at the partial pressure thereof in the annealing atmosphere being capable of forming transient but not stable reaction products on the surface of the sheets, the vacuum being at least 0.1mm. of Hg except for said vapors, and continuing the annealing until the surfaces are bright and substantially complete cube-on-J face grain growth by secondary recrystallization takes place.

3. In .the process of producing silicon-iron sheets hav-' silicon, the balance being iron except for small quantities of additives and incidental impurities at least once to eifect a reduction of from 50% to 90% with intermediate anneals of from 750 C. to 1000 C. in a hydrogen atmosphere, to a sheet having a final thickness of from :1 to 30 mls, and subjecting the sheet at the desired final thickness to a final anneal at a temperature of from .1100" C. to I425 C. in an atmosphere which the initial "stages of the annealing will produce on the sheets a bright metallic surface free from any continuous films, said atmosphere containing during at least the last stages as an essential component to provide an accelerator for cube-on-face grain growth, a vapor of at least one material reactive with the silicon-iron, the vapor being present at a partial pressure of from 0.1 to 100 mm. of Hg, the vapor at the annealing temperature and at the partial pressure thereof in the annealing atmosphere being capable of forming transient but not stable reaction products on the surface of the sheets, said vapor comprising at least one halogen element from the group consisting of chlorine, bromine and iodine, and hydrogen chloride, hydrogen bromide and hydrogen iodide which dissociate at the annealing temperatures, and continuing the annealing until the surfaces are bright and substantially complete cube-on-face grain growth by secondary recrystallization takes place.

4. In the process of producing silicon-iron sheets havsilicon, the balance being iron except for small quantities of additives and incidental impurities at least once to eifect a reduction of from 50% to 90% with intermediate anneals at from 750 C. to 1000 C. inv a hydrogen at-.

mosphere, to a sheet having a final thickness of from 0.1 to mils and subjecting the sheet at the desired final thickness to a final anneal at a temperature of from 1100 C. to 1425 C. in an atmosphere which during at least the initial stages of the annealing will produce on the sheets a bright metallic surface free from any continuotls films, said atmosphere containing as an essential component to provide an accelerator for cube-on-face grain growth, a vapor of at least one material reactive with the silicon-iron which at the annealing temperature and at the partial pressure thereof in the annealing atmosphere is capable of forming transient but not stable reaction products on the surface of the sheets, said vapor comprising at least one halogen element frorn the group consisting of chlorine, bromine and iodine, and hydrogen chloride, hydrogen bromide and hydrogen iodide which dissociate at the annealing temperatures, the partial pressure of said vapor being in the range of from 0:1 to 100 millimeters of mercury, and continuing the annealing untilthe', surfaces "arebright, and substantially complete cube-on-face grain growth by secondary recrystallization "ing a high propotrion of cube-on-face grains, the steps comprising cold rolling a plate of a thickness of from about 0.100 to 0.500 inch of an alloy of from ll% to 10% silicon, the balance being iron except for small quantities of additives and incidental impurities, at least once to effect a reduction of from to with intermediate anneals at from 750 C. to 1000 C. in a hydrogen atmosphere, 'to a sheet having a final thickness of from 0.1 to 30 mls, and subjecting the sheet at the desired final thickness to a final anneal at a temperature of from 1100 C. to 1425 C. in an atmosphere which during at least the initial stages of the annealing will produce on the sheets a bright metallic surface freefrom any continuous films, said atmosphere containing hydrogen gas free from oxygen, water vapor and other oxidizing impurities such that its dew point is less than 40 C. and during at least the last stage of the annealing contains as an essential component to provide an accelerator for cube-on-face grain growth, a vapor of at least one material reactive with the silicon-iron which at the annealing temperature and at the partial pressure thereof in the annealing atmosphere is capable of forming transient but not stable reaction products on the surface of the sheets, said vapor comprising a least one halogen element from the group consisting of chlorine, bromine and iodine, and hydrogen chloride, hydrogen bromide and hydrogen'iodide which dissociate at the annealing temperatures, the partial pressure of said vapor being in the range of from 0.1 to

millimters of mercury, and continuing the annealing until the surfaces are bright and substantially complete cube-on-face grain growth by secondary recrystallization takes place. a

References Cited in the file of this patent UNITED STATES PATENTS OTHER, REFERENCES Journal of Applied Physics, supplement to vol. 31, No. 5, May 1960, pages 4088-4098.? 

1. IN THE PROCESS OF PRODUCING SILICON-IRON SHEETS HAVING A HIGH PROPORTION OF CUBE-ON-FACE GRAINS, THE STEPS COMPRISING ANNEALING COLD ROLLED SHEETS OF A THICKNESS OF FROM 0.1 TO 30 MILS OF SILICON-IRON COMPRISING FROM 1% TO 10% SILICON, THE BALANCE BEING ESSENTIALLY IRON EXCEPT FOR SMALL ADDITIONS AND INCIDENTAL IMPURITIES AT A TEMPERATURE OF FROM 1100*C TO 1425*C. IN AN ATMOSPHERE WHICH DURING AT LEAST THE INITIAL STAGES OF THE ANNEALING WILL PRODUCE ON THE SHEETS A BRIGHT METALLIC SURFACE, FREE FROM ANY CONTINUOUS FILMS, SAID ATMOSPHERE CONTAINING AS AN ESSENTIAL COMPONENT TO PROVIDE AN ACCELERATION FOR CUBE-ON-FACE GRAIN GROWTH, A VAPOR OF AT LEAST ONE MATERIAL REACTIVE WITH THE SILICON-RION WHICH AT THE ANNEALING TEMPERATURE AND AT THE PARTIAL PRESSURE THEREOF IN THE ANNEALING ATMOSPHERE IS CAPBLE OF FORMING TRANSIENT BUT NOT STABLE REACTION PRODUCTS ON THE SURFACE OF THE SHEETS, SAID VAPOR COMPRISING AT LEAST ONE HALOGEN ELEMENT FROM THE GROUP CONSISTING OF CHLORINE, BROMINE AND IODINE, AND HYDROGEN CHLORIDE, HYDROGEN BROMIDE AND HYDROGEN IODIDE WHICH DISSOCIATE AT THE ANNEALING TEMPERATURES, THE PARTIAL PRESSURE OF SAID VAPOR BEING IN THE RANGE OF FROM 0.1 TO 100 MILLIMETERS OF MERCURY, AND CONTINUING THE ANNEALING UNTIL THE SURFACES ARE BRIGHT AND SUBSTANTIALLY COMPLETE CUBE-ON-FACE GRAIN GROWTH BY SECONDARY RECRYSTALLIZATION TAKES PLACE. 